1. Field of the Invention
The invention relates to a proximity fuse for an artillery projectile, particularly projectiles of the type having reduced aerodynamic resistance of the base; comprising a radar arrangement for supplying at the output of a mixer a beat difference signal between a transmitted wave and a reflected wave to at least one first processing chain for activating the trigger device of the ignition circuit of said fuse at the end of a variable time .tau..sub.1 of the order of a tenth second after the said amplified and filtered beat signal has exceeded a given number of times a given threshold.
2. Description of the Related Art
In order to enlarge the range of artillery shells a reduction of the air resistance of the base (RDTC, short for Reduction De la Trainee de Culot) is brought about, which resistance occurs during flight at the base of the shell, especially at the beginning of the trajectory. This reduction of resistance can be obtained by the emission of a gas jet at the base (designated as "base bleed"), of which the principle is explained below. The invention is applicable to a proximity fuse for this type of artillery projectile.
The proximity fuses comprise an electronic circuit sensitive to signals lying in predetermined frequency bands. This circuit responds either to signals transmitted from the allotted target or to reflected signals initially transmitted from the fired projectile, such as especially Doppler signals, the latter case being more particularly envisaged by the invention, which relates to shells, whose privileged target is the ground.
The major technical problem which arises for proximity fuses of this type is the risk of being influenced by parasitic signals, which can produce a premature ignition generally far from the target. In order to suppress these parasitic signals, a known means consists in that a first processing chain is adjoined by a second chain, whose main function is to inhibit the chain in the presence of parasitic signals, as described, for example, in French Pat. No. 2,175,810 and its Addition Pat. No. 2,223,658.
The parasitic signals which justify the presence of the second chain may be due to particular atmospheric conditions or to arbitrary or non-arbitrary disturbances and their frequency bands are situated essentially outside the Doppler frequency band, to which the proximity fuse has to be sensitive during normal operation. For the projectiles of the RDTC type, on the contrary, the additional disturbance induced by the emission of ionized gas at the base has two characteristics which render it very detrimental to a correct operation of the fuse: about half of the additional parasitic radio-electric energy is situated in the Doppler frequency band to be envisaged and the amplitude of these additional parasitic signals is comparable with that of the Doppler signals. With respect to such parasitic signals, the second anti-disturbance chain mentioned above is inoperative.